After spending the bulk of the 1935–36 and 1936–37 seasons in the minor leagues with the Boston Cubs of the Can-Am League, Dumart made the Bruins for good in early 1937. Reunited with Schmidt and Bauer, the trio become one of the most famous lines in hockey history. Dumart - at 6'1", one of the largest wingers of his day - was the skilled checking and defensive component to the line, while contributing good scoring, and helped lead the Bruins to Stanley Cup victories in 1939 and 1941. His contributions were recognized by being named the left wing on the Second All-Star Team in both 1939–40 and 1940–41.

Then World War II intervened - leading to the line being renamed, briefly and abortively, the "Kitchener Kids" due to anti-German sentiment - and Dumart enlisted with teammates Schmidt, Bauer and Frank Brimsek. Joining the Royal Canadian Air Force halfway through the 1941–42 season, Dumart joined the Ottawa RCAF Flyers hockey team which challenged for the Allan Cup, Canada's senior league championship, and scored over a goal a game in leading the team to the title. He played briefly in the fall of 1942 for the Flyers before being shipped overseas, where he served until the end of the war.

Dumart returned in 1945 and played nine more seasons for Boston, and was named a Second Team All-Star for the third time in 1947.

His scoring skills diminishing in his final years, he ended his NHL career with Boston after the 1954 playoffs. He played one last stint the following season with the Providence Reds of the American Hockey League, suiting up for fifteen games before hanging up his skates at last.